1. Field of the Invention
The invention relates to micromachined thermal flowmeters for measuring the flow rate of a flowing fluid, e.g. a liquid or a gas. More particularly, the present invention is directed to a micromachined thermal flowmeter having the thermal sensing means formed in a thin single crystal silicon island, which the crystal silicon island which the crystal silicon is jutted into the flow of the fluid to be measured for increasing the sensitivity and accuracy.
2. Prior Art
In general, a micromachined thermal flowmeter is operated based on the principle of a well-known, hot-wire anemometer and fabricated by using modem silicon integrated circuit (IC) technology. Such a flowmeter offers many advantages including small size, low input power, high sensitivity, fast response, ability for integration, and easiness for batch production. The flowmeters have found an ever-increasing variety of applications such a, for instance, process control in the chemical or semiconductor industries, air conditioning and building control, combustion control in engines and furnaces, and medical measurements.
Over the last ten years several types of the micromachined thermal flowmeters have been developed.
In the first type, a thermopile gas flowmeter uses a thin single crystal member structure micromachined in a silicon substrate for providing high thermal isolation, as shown in FIG. 1. A heating resistor (104) is disposed in the central region of the membrane (102). A thermopile consists of 20 aluminum/polysilicon thermocouples (105) placed on the membrane (102). The xe2x80x9chotxe2x80x9d contacts are positioned near the heating resistor (104) at the tip of the membrane (102), the xe2x80x9ccoldxe2x80x9d contacts are located on the bulk silicon (101). The flowmeter also comprises a passivation layer (103) and a metallization pattern including bonding pads (106).
This type of flowmeter suffers the following problems:
(1) The thin membrane of the flowmeter is easy to damage under the conditions of higher air flow loading and bombardment of particulate matter.
(2) The fluid flow to be measured is easy to be disturbed by the opening on the surface of the membrane adapted to allow the fluid passing over.
(3) The flowmeter cannot be used for liquid because the liquid filled in the opening would reduce the thermal resistance between the cantilever beam and the bulk silicon.
(4) The flowmeter cannot be used in corrosion environment, because the back side of the thin membrane has no protecting layer thereon.
In the second type, a flowmeter has an air flow opening micromachined in a silicon substrate (201) by anisotropic chemical etching, and bridged by two beams (202A and 202B), as shown in FIGS. 2A and 2B. Each beam has a nickel film resistor (204) along its length, electrically isolated form the underlying silicon by a SiO2 layer (203) and passivated by a Si3N4 layer (205), but thermally closely coupled to it. Aluminum leads make contact with these resistors and connect to four bonding pads (206) on one edge of the chip. On beam (202A) is heated via its resistor by means of a control circuit. The other beam (202B) is unheated and serves as an ambient-temperature reference for temperature compensation.
With this type of flowmeter, the above mentioned problems (2), (3), and (4) remain to be solved. In addition, large cross-section area of the beams degrades the performance of the flowmeters such as sensitivity and response time.
In the third type, a flowmeter, as shown in FIG. 3, is made of a silicon substrate (301) having a central circular region and an outer annular region on one side and a cap (306) having two cavities (307) and (308). A heating element (303) is disposed in the central circular region and at least one thermometer component (304) is disposed in the outer annular region of the silicon substrate (301). The two regions are insulated from each other by a ring region of oxidized porous silicon (305). The flowmeter is adapted to receive the flow of fluid over side of the substrate which has a micromachined cavity (302).
This type of flowmeter also has several problems.
Firstly, the manufacturing process of the flowmeters involves two substrate-processing and then bonding the two substrates together with specific alignment and bonding tools. This complicated process increases cost greatly.
Secondly, the oxidized porous silicon has thermal expansion characteristics that are different form the silicon. Due to the thermal stress, the devices disposed near the oxidized porous silicon are easy to degrade if the change in the operation temperature is too large.
Thirdly, the recesses of the cap substrate prevent the device substrate from thinning out to a small thickness. If the thickness of the device substrate is less than the depth of the oxidized porous silicon region, the lateral thermal isolation between the central region and the outer annular region cannot be realized very well.
Fourthly, since the cap substrate covers the front surface of the device substrate, it is difficult to adapt an electrical connection to the external circuit.
An object of the present invention is to provide a micromachined thermal flowmeter in which the heating element and the temperature sensing element are formed in thin crystal silicon islands that are jutted into the flow of a fluid to be measured for achieving higher sensitivity and accuracy, faster response, lower input power, and more roust structure.
Another object of the present invention is to provide a micromachined thermal flowmeter in which the heating element and the temperature sensing element do not contact with the fluid to be measured in order to avoid them suffering problems of abrasion, corrosion, and contamination.
Still another object of the present invention is to provide a micromachined thermal flowmeter which the fluid to be measured flows over a flat surface without any recesses in it in order to avoid disturbing the flow.
Still another object of the present invention is to provide a micromachined thermal flowmeter in which the surface faced with the fluid to be measured can be coated with a corrosion-resistance and abrasion resistant layer in order to prolong the period of the operation of the flowmeter.
Still another object of the present invention is to provide a micromachined thermal flowmeter than can be fabricated by using standard integrated circuit technology with a small modification to achieve high cost-effectiveness.
The above and other objects are achieved by a micromachined thermal flowmeter in accordance with the present invention.